This invention relates to a static pressure measuring or activating device which measures or is responsive to air flow through a duct, pipe or tube using pitot tubes and a plate to amplify the differential static pressure for more accurate measurement.
Pitot tubes are commonly used to measure the stagnation pressure of a flowing fluid such as a gas or liquid. When only one pitot tube is used, the open end of the pitot tube is conventionally directed into the fluid and connected to a pressure-indicating device, such as a pressure gauge. When two pitot tubes are used in combination in a duct containing gas flow, the instrument connected to the pitot tubes may be used to measure the differential static pressure drop and thereby the volume of gas flowing through a duct.
A typical pitot tube static pressure measurement device includes two pitot-type tubes each having a proximal end connected to a pressure transmitter, gauge, switch or the like and a distal end portion extending into the duct generally perpendicular to the gas flow. In one embodiment known in the art, the distal open ends of the tubes are bent or turned 90 degrees, such that the open end of one pitot tube is directed into the gas flow and the open end of the other pitot tube is directed downstream or away from the gas flow.
In certain applications, it is necessary to accurately measure or control the flow of gas through a duct. One application disclosed herein is the control of gas fired burners used to indirectly heat a paint drying oven containing volatile organic compounds which are combustible. The heated gas from the gas fired burner is directed into a heat exchanger and the air in the paint oven circulated through the heat exchanger. The National Fire Protection Association (NFPA) Code requires that the burner be shut off in the event that gas (air) is not flowing through the exhaust duct which receives the air from the heat exchanger and vents the air to atmosphere. In this application, it is necessary to continuously monitor the gas flow through the exhaust duct and shut off the gas burner in the event that the flow of gas through the exhaust falls below a predetermined minimum to avoid a hazardous condition. Conventional pitot tube differential static pressure instruments do not have sufficient accuracy for this and other similar applications and a fixed annular orifice plate is installed in the exhaust duct to amplify the pressure drop for more accurate reading. As will be understood, the installation of a fixed annular orifice plate in the exhaust ducts of a conventional paint drying oven requires materials and labor, particularly where the paint oven includes several heat zones each requiring a gas burner, heat exchanger, exhaust duct, etc.
It would therefore be desirable to improve the accuracy of a conventional pitot tube differential static pressure measurement device to improve its sensitivity and thereby its accuracy.
As set forth above, the present invention relates to a differential static pressure measuring or actuating device measuring or controlling gas flow through a duct, pipe, tube or the like. As used herein, the term xe2x80x9cductxe2x80x9d is intended to broadly cover any passage conveying a gas, such as a pipe, tube or duct, wherein it is necessary or desirable to measure or control the flow of gas through the passage. The differential static pressure measuring or actuating device of this invention includes two tubes, commonly referred to as xe2x80x9cpitot tubes,xe2x80x9d each including a proximal end located outside the duct and a distal end portion located in the duct, wherein the distal end portions of the tubes extend generally parallel and perpendicular to the air flow through the duct each having an open distal end. A plate is located adjacent to and between the distal end portions of the tubes and the plate extends beyond the open distal ends of the tubes creating a differential static pressure or pressure drop between the open distal ends of the tubes. A pressure gauge, pressure transmitter or pressure switch is then connected to the proximal ends of the tubes which may be connected to a control, such as a control for a gas fired burner, fan or the like. As will be understood, a pressure transmitter or switch will also include a pressure gauge.
In the preferred embodiment, the open distal ends of the tubes are inclined toward the plate, most preferably toward a distal end of the plate spaced from the open distal ends of the tubes, preferably at an angle of between of 30 and 60 degrees such that the open end of one tube generally faces the gas flow and the open distal end of the other tube faces away from the gas flow. Although the shape of the plate is not believed to be critical, a round plate has been found to be particularly suitable, wherein the distal open ends of the tubes are adjacent the center of the circular plate and the tubes contact the plate. Again, the size of the plate is not considered critical. However, the plate should not interfere with the flow of gas through the duct.
As understood, the improvement in sensitivity and therefore accuracy results from the plate or disk amplifying the differential static pressure between the open distal ends of the tubes. Further, the plate averages out turbulence in the duct. Also, the differential static pressure measuring or activating device of this invention is less expensive to manufacture than conventional pitot tube instruments described above, wherein the distal ends of the tubes are bent outwardly and must be aligned with the gas flow, as described. Further, in the application described above, the fixed annular orifice plate may be eliminated resulting in a more substantial saving in cost in this application. Other advantages and meritorious features of the differential static pressure measuring or activation device of this invention will be more fully understood from the following description of the preferred embodiments, the appended claims and the drawings, a brief description of which follows.